Method and apparatus for driving reflective bistable cholestric displays

Abstract

This invention provides a method and apparatus for driving bistable cholesteric liquid crystal displays. The method and apparatus provided a display in which all the pixels are initially driven to the P state. Selected pixels for the display are then driven to the FC state to provide the desired message. The state of the pixels is then maintained for a viewing period prior to any resetting of the display to the P state. The switching from the P state to the FC state allows fast addressing times and lower voltages to be used in driving the display.

What is claimed is: 
     
       1. A bistable cholesteric liquid crystal display comprising:
 a plurality of pixels within said display; 
 driving means to apply voltage to each pixel; and 
 control means for controlling said driving means to supply a refresh pulse to said pixels to set all pixels to the reflective planar (P) state, to subsequently scan the plurality of pixels and supply a selection pulse to selected pixels to switch said selected pixels to the scattering/transparent focal conic (FC) state within about 0.5 ms to 2 ms without utilizing a surfactant within the bistable cholesteric liquid crystal display and to maintain said display for a period of time for viewing of the display. 
 
     
     
       2. A bistable cholesteric liquid crystal display as claimed in  claim 1  wherein said display includes a matrix of overlapping electrodes with the pixels of the display being defined by overlapping regions of said matrix of electrodes. 
     
     
       3. A bistable cholesteric liquid crystal display as claimed in  claim 2  wherein said matrix of overlapping electrodes comprises a first set of electrodes and a second set of electrodes with the pixels defined by the overlapping regions between said first and second sets of electrodes and wherein the refresh pulse from the driving means is provided to said electrodes to drive all said pixels to the P state. 
     
     
       4. A bistable cholesteric liquid crystal display as claimed in  claim 3  wherein the voltage supplied to the pixels for the display comprises providing an address voltage to one set of electrodes and a data voltage to the remaining set of electrodes such that selected pixels receive a cumulative total of said voltages and non-selected pixels receive a data voltage of opposite sign to provide a lower total voltage to non-selected pixels. 
     
     
       5. A bistable cholesteric liquid crystal display as claimed in  claim 4  wherein said address pulse voltage is approximately 24±3V. 
     
     
       6. A bistable cholesteric liquid crystal display as claimed in  claim 4  wherein said data pulse voltage is approximately 6±2V. 
     
     
       7. A bistable cholesteric liquid crystal display as claimed in  claim 1  wherein said electrodes comprise transparent conductive film. 
     
     
       8. A bistable cholesteric liquid crystal display as claimed in  claim 7  wherein said transparent conductive film comprises indium tin oxide. 
     
     
       9. A bistable cholesteric liquid crystal display as claimed in  claim 1  wherein said liquid crystal cell has a gap of 4 to 20 μm. 
     
     
       10. A bistable cholesteric liquid crystal display as claimed in  claim 1  wherein the liquid crystal display is driven only on receipt of a signal by the control means. 
     
     
       11. A bistable cholesteric liquid crystal display as claimed in  claim 1  wherein said display is incorporated in a pager or cellular telephone. 
     
     
       12. A method of driving a bistable cholesteric liquid crystal display comprising the steps of:
 providing a refresh pulse to set all pixels to the reflective planar (P) state; 
 switching selected pixels to provide the desired pattern to the scattering/transparent focal conic (FC) state within about 0.5 to 2 ms without utilizing a surfactant within the bistable cholesteric liquid crystal display; and 
 holding said display for a suitable viewing period. 
 
     
     
       13. A method of driving a bistable cholesteric liquid crystal display as claimed in  claim 12  wherein said method comprises providing electrical pulses to column and row electrodes to impart a voltage on the cholesteric liquid crystal material in each pixel to drive the switching between the P state and the FC state. 
     
     
       14. A method of driving a bistable cholesteric liquid crystal display as claimed in  claim 13  wherein said refresh pulse is in the range of 10 to 40 V. 
     
     
       15. A method of driving a bistable cholesteric liquid crystal display as claimed in  claim 13  wherein said step of providing electrical pulses to electrodes to switch said selected pixels to the FC state to provide said display comprises driving one set of electrodes with an address pulse and the remaining set of electrodes with data pulses such that the selected pixels are subjected to a voltage being the sum of the address and data pulses which is greater than the threshold voltage to switch to the FC state and the non-selected pixels receive a voltage with a data pulse of inverse sign at a voltage below the threshold for switching to the FC state. 
     
     
       16. A method of driving a bistable cholesteric liquid crystal display as claimed in  claim 13  wherein said viewing phase comprises applying insufficient voltages to any pixels to cause a change from the P state to the FC state. 
     
     
       17. A bistable cholesteric liquid crystal display for switching pixels between a first stable state and a second stable state, comprising:
 a plurality of pixels; 
 driving means for applying voltage to each pixel; and 
 control means for controlling said driving means to supply a refresh pulse to said plurality of pixels to place said plurality of pixels in a first stable state, to subsequently scan the plurality of pixels and supply a selection pulse to selected pixels of said plurality of pixels in the first stable state to switch said selected pixels to a second bistable state within about 0.5 ms to 2 ms without utilizing a surfactant within the bistable cholesteric liquid crystal display to provide a desired pattern, and maintaining said display in the second bistable state for a period of time for viewing of the display; 
 wherein said first stable state is the Planar (P) state and said second stable state is the Focal Conic (FC) state.